The typical epicyclic gear system has a sun gear, a ring gear surrounding the sun gear, and planet pinions located between and engaged with the sun and ring gears, and in addition, it has a straddle-type carrier that provides pins about which the planet pinions rotate, with the pins being anchored at both ends in the carrier. A gear system so configured has the capacity to transfer a large amount of power in a relatively compact configuration—or in other words, it has a high power density.
But heavy loads tend to distort the carrier and its pins and skew the axis about which the planet pinions rotate. Under such conditions, the planet pinions do not mesh properly with the sun and ring gears. This causes excessive wear in the planet pinions and the sun and ring gears, generates friction and heat, and renders the entire system overly noisy.
A planetary system in which the planet pinions are supported on and rotate about so-called flexpins mitigates the skewing. In this regard, a flexpin for a planet pinion at one end is anchored in and cantilevered from the wall of a carrier of which it is a part. The other end of the flexpin has a sleeve fitted to it, with the sleeve extending back over, yet otherwise spaced from the flexpin. The sleeve supports the planet pinion, in that it serves as a component of a bearing for the pinion. In other words, flexpin technology employs a double cantilever to offset the skewing that would otherwise occur. See U.S. Pat. Nos. 6,994,651 and 7,056,259, which are incorporated herein by reference, for a further discussion of flexpin technology.
The cantilevers produce high stresses in the flexpins, and to have more moderate stresses, some carriers have two walls with flexpins anchored in each of the walls and, of course, a separate planetary pinion around each flexpin. This doubles the number of flexpins to share the torque transferred through the system and thus reduces the unit load applied to each flexpin. The planet pinions are arranged in two arrays between the walls, there being for each pinion in the one array and corresponding pinion aligned with it in the other array. Spaces exist between pairs of corresponding pinions and webs extend between the two walls in these spaces. The carrier, whether it rotates or not, is subjected to an externally applied torque at one of its walls. The planet pinions transmit torque through the system, but the lengths of the load paths from the flexpins on the two walls differ, the load paths from the flexpins on the primary wall, which is subjected to the external torque, being considerably shorter than the load paths from the flexpins on the other or secondary wall. This renders the array, identified with the shorter load paths stiffer than the array identified with the longer load paths. The carrier undergoes a distortion that causes the flexpins on the secondary wall displace angularly with respect to the flexpins on the primary wall, reference being to the axis of the planetary system. Since the planet pinions of the two arrays mesh with the sun and ring gears, the displacement causes an uneven sharing of the torque transmitted at the teeth where the pinions mesh with the sun and ring gears.